Wireless direct sequence spread spectrum digital cellular telephone system

ABSTRACT

The present invention is based on novel implementation techniques which makes orthogonal CDMA practical in a short range mobile telephone environment where significant multipath fading exists. Specifically, this invention provides novel techniques for establishing the time base, frequency, and power control necessary to achieve orthogonality. Use of a high power sounding burst on the outbound link permits: 1) antenna diversity selection to minimize the probability of a faded condition, 2) local frequency locking at the subscriber terminal which avoids the requirement for a costly precision frequency standard, and 3) essentially instantaneous inbound power control based on the outbound receive signal level. This is effective since time division duplexing is used and both transmission and reception take place on the same frequency. With the short frame structure and unique placement of the sounding burst the correlation between the outbound and inbound path losses is very high. Thus, according to the invention, the signal structure and control algorithms result in a greatly reduced signal level range at the base station/PBX to achieve high efficiency in a orthogonal CDMA system. Real world effects such as filtering, multipath time spread, and time base error destroy orthogonality and introduce a degree of cross coupling between supposedly orthogonal channels. Thus, the invention provides accurately controlled power levels in this highly dynamic environment.

DESCRIPTION OF PRIOR ART

Spread spectrum communication is characterized by modulation schemeswhich greatly expand the bandwidth occupied by a voice or datainformation signal. The two most frequently used schemes are directsequence spreading and frequency hopping. In direct sequence frequencyspreading, which is employed in this invention, a digitized informationsignal typically modulates a pseudo-random (also referred to aspseudo-noise or PN) digital signal. If the bit rate of the PN signal is,say 32 times as large as that of the information signal, the bandwidthof the resulting modulated signal becomes 32 times that of the originalinformation.

The key to receiving spread spectrum signals is a receiver capable ofgenerating a second PN signal identical to that used to spread thetransmitted signal's bandwidth. This is possible because bothtransmitter and receiver use identical random digital sequence (PN)generator circuits. The PN signal is used by the receiver tosynchronously demodulate the received signal. To do this successfully,the time-variations of the PN signal must be in synchronism with thosein the received (modulated) signal. If they are not in time synchronism,the detected signal will be minuscule. Traditionally the time-phase ofthe PN signal generator at the receiver is varied slowly in time untilsignal output is found to be a maximum, and kept locked to the phase ofthe transmitter's PN generator by a phase-locked loop circuit.

An important capability of spread-spectrum communication, also used inthis invention, is code-division multiple access. This involves carryingon a multiplicity of communications simultaneously, in the samebandwidth and geographic area, by using different time-varying PN codeswhich define each independent communication "channel".

Diversity reception is a well-known technique wherein several receivingantennas are used in connection with one or more receivers and some formof manual or automated antenna switching. The object of such schemes isto overcome fading in propagation paths between transmitter andreceiver, by selecting the signal from that antenna (or combination)whose received signal is strongest at any given instant.

SUMMARY OF THE INVENTION

The wireless telephone system of this invention provides for acombination of a base station unit and multiple handsets to provide, inthe embodiment described herein, sixty-two concurrent communicationchannels.

Two operating environments are envisioned: indoor (within buildings) andoutdoor. The operating range in each case will be limited to about 500meters by (U.S.) Federal Communication Commission limits on transmitterpower. Typically the indoor operating range will be on the order of 200meters or less depending on the environment in which the systemoperates. The reduction of operating range is a result of additionalpath loss, which can be experienced due to multi-path fading and/orintervening walls, partitions, or other structures between the handsetand the base station.

One object of the invention is to achieve a wireless telephone systemwhich is both reliable and economically producible. This is accomplishedby the choice of communication techniques and waveform structure, and bythe use of modern application specific integrated circuits (ASICs).

Another objective is to simplify manufacturing procedures and reducecosts through extensive use of digital signal processing techniquesthroughout the system. The use of digital circuits minimizes need forcircuit adjustments, alignment or tuning often required by prior artwireless telephony equipment. In the preferred embodiments, a minimalpart of the circuits are implemented using analog technology.

Still another object is to minimize, in a real-time sense, the effectsof transmission impairments imposed by the operating environment. Thisis implemented through the combination of four specific techniques:

1) Mutual interference between the multiple user signals is minimized byuse of pseudo-noise modulation signals which are orthogonal to oneanother, i.e. which can be independently demodulated.

2) Direct sequence spread spectrum modulation is used to provideprotection against unintentional jamming by ambient narrow-band signalssuch as those from personal computer oscillators. It further protectsagainst other interfering users sharing a common area, and providesusers with a high degree of privacy.

3) Antenna polarization diversity reception is combined with a real-timemeans of selecting the antenna with the best signal-to-noise ratio(SNR).

4) Automatic power control is implemented so that all signals will bemaintained at appropriate levels, thereby controlling mutualinterference due to one communication signal overpowering others, whereuser handsets are located a widely varying distances from the basestation.

Yet another object of this invention is to significantly increase thenumber of user channels in a given area in the allocated bandwidth. Inthe preferred embodiment, each 62-user group channel occupiesapproximately 1.33 MHz. This permits up to 19 base stations to operatewithin communication range of one another without interfering. Thesystem embodiment described provides means to permit a handset user tomove from the area served by one base station to that served by another,with automatic handoff from one to the other.

A further object of the invention is to provide means forinterconnecting users for communication, and for connecting users tostations on remote telephone systems. In the embodiment described, thisis done by connecting each base station to telephone switchingequipment, providing each handset user with separate access to a localdial network, and through that to common carrier networks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a single base station embodiment of the invention inschematic form,

FIG. 2 shows a single handset, along with its removable cradle, arepresentative means to ensure that batteries powering it are maintainedat full charge,

FIG. 3 shows in block diagram form a configuration including amultiplicity of base stations, each supporting in this case 128 handsetsof which 62 may be in use at any given instant,

FIG. 4A illustrates the assignment of (group) channels in a portion ofthe electromagnetic spectrum allocated for use by this type ofcommunication service,

FIG. 4B illustrates the use of alternate channels in a given physicalarea to minimize interference between groups of handsets,

FIG. 5 shows a representative configuration for the handset, with avertically polarized whip antenna mounted at top and a horizontallypolarized loop antenna embedded in its base,

FIG. 6 Portrays one 10 millisecond frame of a preferred overall (orderwire and voice channel) signal structure,

FIG. 7 shows the combination of sub-frames into a 640 millisecondsignal,

FIG. 8 illustrates the detailed signal structure of an order wirechannel,

FIG. 9 is a generic representation of information layout of order wirecommands and the associated response,

FIG. 10 illustrates the location and utilization of control dataembedded in voice transmissions, for system control during communicationbetween a handset and base station,

FIG. 11 is a voice channel control command format and extraction fromvoice channel signals,

FIG. 12 is a general block diagram of the sub-frame synchronizationincorporated in the invention,

FIG. 13 is a block diagram of signal power measurement for antennaselection and transmit power control,

FIG. 14 is a diagram of the frequency discriminator and AFC carriertracking loop incorporated in the invention,

FIG. 15 illustrates a PN code phase discriminator and tracking loop athandset incorporated in the invention,

FIG. 16 illustrates a PN code phase discriminator and tracking loop at abase station,

FIG. 17 is an exemplary block phase estimation and differential datadecoding circuits incorporated in the invention,

FIG. 18 is a diagrammatic illustration of differential data encodingincorporated in the invention, and

FIG. 19 is a block diagram of automatic gain control (AGC) in thehandsets.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the hardware configuration for one 62 user systemhardware set, e.g., basic single base station system configuration. Eachhardware set is comprised of one base station 10 and up to 62 handsets11-1, 11-2 . . . 11-N with cradle. The system defines a star networkconfiguration with the base station as the center of the star. The basestation 10 contains one transceiver 12 for each individual user handsetin the operating system. Polarization diversity is provided in thesystem by using dual cross polarized antennas 11A1 and 11A2 in eachhandset.

A single antenna 13 is used in the base station 10. Only one antenna isrequired because the communication channel is symmetrical with respectto direction, to and from the base station, so that dual cross-polarizedantennas at the handset are sufficient to provide diversity in thesystem. Transceivers 12 are coupled by up/down converter anddistribution amplifiers 14 to antenna 13 and served by a commonreference oscillator 15 clock, logic 16 and telephone system (TELCO)interface 17.

The handset hardware configuration is shown in FIG. 2. The handsetcradle 18 serves two purposes. It provides a place to physically storethe handset 19 when not in use, and it provides a charging capability toreplenish the charge on the handset batteries as required. Red and greenalarm lights 20 are provided on the handset 19. These lights 20 serve toindicate the adequacy of the physical location of the cradle. If thereceived signal strength is adequate, a green light will illuminate. Ifthe received signal strength is not adequate a red light will illuminateand the handset 19 can be moved a few inches. Since the handset containspolarization diversity, the need to relocate the cradle location willalmost never occur.

The primary purpose of the system in this embodiment is to provide voicetraffic capability to the potentially mobile user community. In order toprovide this capability, a telephone system (TELCO) support andinterface capability is provided. This TELCO support functions consistsof 1) call establishment operations support, 2) user information database support and update, 3) multicall programming operations capability,and 4) peripheral support functions.

The present invention provides a practical system for achieving ineffect orthogonal CDMA operation thereby significantly increasing thenumber of user channels in a given area in a given bandwidth as comparedto conventional CDMA operation. This is done by two means: 1) having thebase station measure the time base error of each handset andtransmitting the correction information to the handset, and 2) using"instantaneous" power control of the return or inbound (handset to basestation) link based on the time-division duplex operation, measuring thepower of the pilot or sounding signal, and performing the appropriatecompensation of the handset transmit power, e.g., if the pilot signal isreceived 10 dB too low as compared to the desired reference level thenthe handset transmit power is increased by 10 dB with respect tonominal. If true orthogonality were achievable in practice power controlwould not be a crucial issue. However, truly orthogonal signals, i.e.,those which cannot interfere with each other no matter what the powerlevel difference, do not exist in the practical world with finitebandwidth, filtering effects, and minor time base errors. Thus, there issome cross-talk or interference between our "orthogonal" signals and itis important to control the power of each return signal precisely andquickly so that significant interference does not occur. Thus, theinnovative power control system described herein is a crucial feature ofthe invention.

Another feature of the invention is that the handset locks itsfrequencies to the base station reference frequency by phase-locking tothe powerful pilot or sounding signal. This permits the return linksignal from the handset to have a very precise frequency withoutrequiring the handset to have a costly, high stability oscillator. It isnecessary to control the frequency of the handset signals reasonablyclosely if the signal orthogonality is to be maintained. The systemdescribed herein does this in a very cost-effective manner.

CALL ESTABLISHMENT OPERATIONS

This comprises interfacing with the TELCO, providing and interpretingall signaling operations required to establish both incoming andoutgoing calls. This includes such things as dialing, a busy signal, anda phone ringing operation. All these functions are handled by the orderwire (OW) channel and described in later herein.

BASE STATION CONFIGURATION

A typical multiple base station system configuration is illustrated inFIG. 3. A system of N base stations BS-#1 . . . BS#N each with 62 voicetraffic channel capability is shown. Also shown is that each basestation may be required to support up to 128 (not all in use at once)users (HS#1 . . . HS#128) part time. For these assumed conditions theTELCO (this TELCO unit is sometimes referred to as a MobileTelecommunications Switching Office (MTSO) base station system must havethe capability to recognize and properly route calls to 128n differentphone numbers (different users).

MULTIPLE CELL OPERATION

So long as users are confined to operate through only one particularbase station, operations are well defined and the equipment need concernitself only with maintaining signal timing and appropriate transmitterpower level. If the system is defined to consist of many base stationsover an extended geographical area, or covering multiple floors in amulti-floor building, the user must be able to roam, or execute ahandover operation from one base station to another. Thus, in a multiplebase station system it is assumed that any user can roam from the cellarea serviced by his original base station to the cell area covered byany other compatible base station.

The importance of a cell pattern is threefold: 1) it defines a minimumrange between two cells sharing the same frequency thereby definingco-channel interference effects, 2) it can define the exclusiveneighbors of any given cell thereby reducing the search time for a newcell when attempting a roaming/handover operation, and 3) it defineswhether a multifloor building can be serviced without sufferingsignificant interference between like cells on adjacent floors.

A twelve pattern is very desirable for all these reasons. A hexagonal 12cell pattern has six uniquely defined neighbors per cell and provides a6 cell radii separation between like cells. For multifloor operation,this provides 3 cell radii separation plus the attenuation betweenfloors. For indoor operation it is likely that a square pattern may beused since a square, or rectangular, pattern may lend itself better foruse within a building.

As a user roams about his cell, he will at times reach the boundary ofgood coverage. As the handset realizes it is reaching the limits of itsoperating range, it will identify the cell area he is about to enter.The handset will constantly search for signals from other adjacent usergroups which are members of the total system but outside his presentcell. This will be done by searching for other OW signals than the OW ofhis own cell group. In order to minimize the search time and minimizethe likelihood of losing the presently in use voice channel before hecan establish a new one with the next base station, a handset maintainsa data base defining relative timing between all adjacent base stations.The details of this operation are presented later.

Once the OW of the "next" cell is contacted, the handset must nowrequire admission to the cell as a new user. If admitted, the handset isassigned an identification number as an authorized user of the group. Atthis time all pertinent data on the handset, i.e., handset serialnumber, identification number, and telephone number must be relayed toand stored in the base station database. The local TELCO data base mustalso be updated so that it knows where, i.e., to which base station, todirect calls intended for that particular telephone number. If a call isin progress, handover now involves the local TELCO intimately. The localTELCO must now not only have its data base updated, it must re-route acall in progress from one base station to another in real time.

The system is limited by FCC rule to operating with no more than 1 watt(30 dBm) transmitted power from either the handout or the base station.Based on this, the base station is clearly the limiting factor. However,according to the invention, a very viable system can be set up whilesatisfying the 1 watt total maximum power limitation. In general whenservicing a densely populated user community high capacity base stationscapable of servicing a large number of users can be employed and willoperate over a relatively short range. Alternately, when servicing asparsely populated user community, lower capacity base stations capableof servicing a smaller number of users can be utilized operating over agreater communication range.

AUTOMATIC GAIN CONTROL

USER TO BASE STATION

Each base station transmits a reference signal at a fixed level againstwhich all estimates of received handset signal levels are compared. Onthe basis of these comparisons, the transmit power of each handset isadjusted as described later. The power control system can maintain thepower received at the base station from each handset to within anaccuracy of about 1 dB even in presence of severe multipath fading.

BASE STATION TO USER

The base station transmit power level is held fixed at the maximum powersetting. As a handset is transported throughout the cell, its receivedsignal level will vary over a maximum dynamic range of about 90 dB. Inorder to maintain the input voltage to the main signal pathanalog-to-digital converter in the user unit at nominally half of fullscale, and thereby avoid clipping and loss-of-resolution problems, anAGC function is implemented prior to the analog-to-digital.

FREQUENCY PLAN

The system RF frequency plan for the disclosed embodiment is illustratedin FIG. 4. The FCC rule 15.247 band intended for this type ofapplication extends from 902 MHz to 928 MHz, providing a 26 MHz totalsystem bandwidth. Each subgroup signal is allocated a 1.33 MHzbandwidth. The frequency spacing between adjacent subgroup carrierfrequencies is set to 0.663 MHz. This is possible since precise chiptiming is maintained such that orthogonal operation is possible evenwith substantial spectral overlap. Thus, a total of 38 subgroups can beaccommodated.

The system provides the feature that different PN sequences may be usedin different cells. The use of different PN sequences in neighbor cellsminimizes co-channel interference. Different PN sequences would be usedin neighbor cells when a given cell configuration forces neighbor cellsto be placed closer to each other than desired.

POLARIZATION DIVERSITY

Antenna polarization diversity at the user handset is selected, in thepreferred embodiment, as the most effective method to reduce multipathfading. Implementation of polarization diversity at the handset requirestwo antennas at the handset and a single switch to select between them.Channel sounding is performed in order to select the best antenna, ineach 10 ms time subframe.

Studies conducted indicate that polarization diversity provides animprovement in signal reception capability as good as or better than anyother diversity technique. The use of polarization diversity does notimpact system capacity as some techniques do and, the additionalhardware complexity required to add polarization diversity is minimal.The system implements the use of dual cross polarized antennas at thehandset. A typical handset antenna configuration is illustrated in FIG.5. The antenna configurations shown in FIG. 5 makes use of a whip 11A1and an Alford loop 11A2. Separation of whip 11A1 and loop 11A2 maycompromise polarization diversity performance but will then providespatial diversity. In the preferred embodiment, the loop should beapproximately 3 inches square to have the same sensitivity as whipantenna 11A1.

The base station antenna pattern should be appropriate to the area to beserved. If the Base Station is located in the center of the service areaits pattern should be omnidirectional in the horizontal plane. In mostcases, the user will be distributed over a narrow vertical span and theBase Station antenna can have a narrow vertical pattern. Such patternsare ordinarily obtained by the use of vertical linear arrays. Aconvenient element for such an array is the Lindenblad radiator inventedin 1936 for use at 120 MHz. It is an assembly of four dipoles spacedaround a center support post; tilted at 45 degrees, and fed in phase.

This antenna provides a circular polarized wave. An array of theseelements can easily be assembled to narrow the vertical pattern, with apractical limit imposed by the space available for mounting. Thisassembly has been used commercially. The advantage of the Lindenbladdesign is that it is simple and very tolerant of implementationvariations.

In the event the user distribution is wide in the vertical direction--asfor several floors in a tall building, a less directive antenna would bedesired. Then a single element or short array would be preferred.

MULTIPLE BASE STATIONS: SYNCHRONIZATION

When two handsets operating in two mutually adjacent cells (served bydifferent base stations) find themselves near each other and at the cellboundary, an adjacent channel interference (ACI) ratio of I/S=80 dB ormore can result. If the two cell systems are not synchronized, and ifone handset is transmitting while the other is receiving, operations atboth handsets will be disrupted. This can be avoided by making adjacentbase stations mutually synchronous to an accuracy of ±8 μs. This is sobecause there is a 16.6 μs minimum gap time between successivereceive/transmit time intervals in each subframe.

The preferred timing approach in this disclosed embodiment is to provideinput from a precision timing source to a central site (one of the basestations (FIG. 3) is designated to be Master base station). This timingsignal can then be distributed to a constellation of base stations alongwith the other TELCO interface lines. This approach applies to bothindoor and outdoor base station systems. In an indoor system there wouldbe one Master base station or central site. In an outdoor system therecould be many depending on the extent of the system and itsconfiguration.

Synchronization for a limited system, for example, a system intended toservice one building, is not a problem. One base station can bedesignated as the Master station and it would distribute timing to allother base stations. The timing signal can be distributed along with theTELCO interface wiring. Alternatively, the GPS, local telephone companycentral office time source, etc. can be used.

SIGNAL STRUCTURE, DATA CONTENT, PROTOCOLS, AND SIGNAL PROCESSING

In this embodiment of the invention, the signal structure for the systemis predicated on two underlying objectives:

(1) to operate synchronously with 20-msec frames of a 16 Kbps voiceencoder/decoder, and

(2) to keep added signal path delays to under 10 msec.

Accordingly, the preferred signal structure is a sequence of 10-msecsubframes, as shown in FIG. 6, each consisting of four distinct periods,two for inbound and two for outbound signalling, and each being one of64 subframes composing a 640-msec frame as shown in FIG. 7. The inboundsignals are spread with a different PN code than the outbound signalsbut with the same code length and chipping rate.

The voice channel data consists of 16 Kbps bidirectional digital voice,plus a 400 bps bidirectional control link. The data modulation isdifferentially encoded QPSK, transmitted at a burst rate of 20.72 Kps.The data signal is bi-phase modulated with a spreading code at 32 timesthe burst symbol rate (663 KHz). The spreading code is the modulo-2 sumof a length-255 PN sequence and a length-32 Rademacher-Walsh (R-W)function. The all-ones R-W function is used as an order-wire channelwithin each 32-channel subgroup; the remaining 31 functions are eachassociated with a different voice channel in that subgroup.

From the perspective of a handset already associated with a particularbase station, the four time periods within each subframe may be viewedas follows:

Throughout this discussion, the term "symbol" is used to mean "voicechannel symbol duration", i.e., 32 chip times, even when the activity ison the order wire channel. The term "voice channel" means one frequencychannel and non-unity Rademacher-Walsh code combination.

(1) (SOUND) The base station transmits a 121/4 symbol all-ones soundingpattern (i.e., no data transitions) on each order wire channel, at alevel 15 dB higher than for individual BS→HS voice channels; eachhandset receives the first six symbols on one antenna A1, switches tothe other antenna A2 during the next 1/8 symbol, receives the next sixsymbols on A2, compares the power between A1 and A2, chooses the antennawith the higher power, and switches to that antenna during the next 1/8symbol.

The power level from the chosen antenna is used by the handset todetermine transmit power during the following HS SYNC and HS→BS portionsof the signal, and also as a code sync error measure to be input to itsdelay-lock code tracking loop.

(2) (BS→HS) On each active voice channel, the base station transmits avoice data burst of 91 QPSK symbols, followed by a guard time of 11chips. The handset receives this data on the antenna selected during thesounding period. The voice channel data is constructed as follows:

1 phase reference symbol

2 channel control symbols

80 encoded voice data symbols

8 spare symbols (reserved for future use)

(3) (HS SYNC) On an automatic cyclic time division multiple access(TDMA) basis, one member handset in each 64 member subcommunity (i.e.,one per order wire channel) transmits a continuous all-ones rangingsignal (i.e., no data transitions but PN chip transitions) to the basestation on its associated order wire channel for a duration of 121/8symbols, followed by a 1/8-symbol guard time. The base station orderwire channel performs a delay lock loop error measurement on thissignal, and prepares and queues a timing correction command, ifrequired, to be sent to that handset at the next opportunity. Eachtransmitting handset transmits using the antenna it selected during thesounding period, at a power level determined from the power received bythat antenna during that period.

(4) (HS→BS) On each active voice channel, the handset transmits a voicedata burst of 91 symbols, followed by a guard time of 11 chips, on theantenna selected

during the sounding period. This inbound burst is of the same format asthe BS→HS burst of period (2).

Thus the time-division duplex signal is symmetrical, with respect toformat and content, its inbound and outbound portions being essentiallyidentical to each other, of the total time available, 77.2% is used forvoice data, 10.6% for related overhead and spare capacity, 5.8% forchannel sounding, 5.8% for handset timing synchronization, and 0.6% forvarious switching and guard times.

Advantages of selected signal structure include:

1) One dedicated bidirectional order wire channel (for link control) foreach 31 voice channels.

2) No voice channel activity during sounding burst (at 15 dB higher thanindividual voice channels, allows very accurate measurements of receivedpower, time offset, and frequency offset.

3) Dedicated handset sync per channel allows accurate measurement ofhandset power and time offset with no interference due to timing errorsin other channels.

4) Bidirectional 400 bps control link incorporated into each voicechannel (for handset power and timing control, as well as link control).

ORDER WIRE CHANNEL SIGNAL STRUCTURE

The order wire channel signal structure is shown in FIG. 8. Four periodsof the overall time-division duplex structure are superimposed on anorder wire signal structure consisting of (in each direction) two OWsymbol periods followed by ten actual OW symbols plus a 7 voice channelsymbol frame sync/parity check signal and a 31-chip guard time. Eachhalf subframe is exactly 13 OW symbol periods in duration.

The order wire signal structure has been designed so as to maximizesignal search effectiveness, i.e., to minimize expected search times.Each OW symbol period=255 PN chips=one PN code sequence length, thus bytaking energy measurements over one OW symbol period, we are integratingover one PN code sequence length and taking full advantage of the PNcode's autocorrelation properties.

Also, the choice of an exact integer number of PN sequence lengths perhalf subframe both 1) greatly simplifies the PN coder design and thesearch algorithm, and 2) is critical to avoiding code phase ambiguitieswhich would increase typical and worst-case initial search times by morethan ten fold.

During the two sounding periods, the switching times allotted at the endof each, and the reference phase period (i.e. for a total of(192+4)*2+118=510 chips=2 OW symbol periods), the base station istransmitting a continuous (spread) tone corresponding to an all-onesdata modulation (i.e. no data transitions). The next 10 OW symbolscontain order wire data, as described below.

The outbound order wire channel frame sync field contains 7 voicechannel symbols (14 bits) organized as 6 bits parity check on the 20 OWbits, 6 bits subframe number within frame (0-63), and 2 bits paritycheck on the subframe number. Thus 12/13=92.3% of the base station orderwire channel transmit time (i.e., 46.1% of the total time) is availableto handsets for signal acquisition purposes.

The inbound order wire signal format consists of two segments. Duringthe first, on a cyclic basis, one handset out of each community of 64transmits a continuous (spread) tone corresponding to an all-ones datamodulation (i.e. no data transitions), for a duration of 388 chips, forthe purpose of allowing the base station to measure that handset'stransmit code synchronization, power, and quality during a periodwherein there is guaranteed to be no interference from other handsets onthe same channel.

Four chips guard time later, if the current order wire time slot isassigned, the handset assigned to this slot transmits first a 118-chipphase reference symbol, then 10 OW symbols, and finally a7-voice-channel-symbol (14-bit) field containing a parity check of the20 order wire bits; the last 31 chips of the inbound order wire signalsegment are merely guard time.

If the current order wire time slot is not assigned, it may be accessedby roaming handsets seeking membership in a new base station community,or by handsets which have just been switched from STANDBY to ACTIVE modeand are seeking a voice channel assignment. The signal structure forsuch accesses is identical to that for assigned accesses.

ORDER WIRE CHANNEL DATA STRUCTURE AND PROTOCOL

Each outbound order wire burst contains a 10-symbol (20-bit) order wirecommand, formatted as shown in FIG. 9. The 5-bit function fieldspecifies which of the various command or broadcast functions is beinginvoked. For most command functions, a 7-bit landset ID field is alsoincluded to specify to which of up to 128 handsets in the local basestation community the command is directed. The remaining 8 bits (or insome cases, all remaining 15 bits) are defined as required by thespecific command or broadcast function.

The response to any outbound (i.e., to a handset) command or inboundrequest which requires a response will be provided in the third halfsubframe following that command or request. Failure to receive a validresponse at that time shall be considered an error and shall causerecovery measures to be taken. Thus, each third half sub-frame followinga base station command requiring a response is defined as beingassigned, and is not available for use by handsets attempting toinitiate communication.

A handset's response to a base station command requiring one is to echothe received command's function and handset ID fields, and follow withwhatever additional meaningful information is required for that command.Thus a handset response generally constitutes a specific acknowledgementof the received command, plus an implied request for the next step inthe dialog leading to the end objective. Similarly, a base station'sresponse to a handset request both acknowledges the request and providesthe next step in the dialog toward the desired objective.

The example diagrammed in FIG. 10 and described below serves toillustrate this:

(1) A base station detects that an incoming call from the TELCOinterface is directed to a handset with the corresponding telephonenumber. It then schedules a Ring Alert command to be sent to thehandset, addressed to it via its 7-bit Handset ID.

(2) On recognizing its ID, the handset responds by echoing tile RingAlert Command and enabling a local "ring" function.

(3) When the user picks up the handset and switches it from STANDBY toACTIVE mode, the handset disables the local ring function and attemptsto reestablish the dialog by issuing an Allocate Channel request in thenext available CSMA slot.

(4) Assuming for the moment that the CSMA Allocate Channel request isreceived properly at the base station (recovery from collisions andother errors is discussed in sections later herein), the base stationechoes the Allocate Channel request to the requesting handset,

(5) which then resubmits it in the now implicitly assigned (i.e.,"guaranteed" collision-free TDMA slot 15 msec later.

(6) Having thus confirmed the Allocate Channel request, the base stationthen allocates a voice channel and issues a channel Assignment commandto the handset,

(7) which echoes the Channel Assignment command in acknowledgement.

(8) Having thus confirmed that the handset has correctly received thechannel assignment information, the base station connects thecorresponding TELCO line to the allocated voice channel and issues aMake Link command to the handset,

(9) which then begins transceiving on the assigned voice channel.

For calls originating at the handset, essentially the same procedurewould be followed, except for steps (1) and (2), which of course wouldbe eliminated.

At the end of any call, the user would switch the handset from ACTIVEback to STANDBY mode, and the handset would signal a Deallocate Channelrequest to the base station via its in-band order wire (or channelcontrol) path (see Section 3.6). This request would be acknowledge bythe base station, via the same path, prior to releasing the channel oneither end.

ORDER WIRE CHANNEL DATA STRUCTURE

Approximately 15 specific order wire channel commands are necessary orvery useful. Some are "broadcast" by the base station on the order wirechannel to indicate network status. Others are involved in initiatingcommunication with a handset, terminating communication, and adjustingtiming. These include:

1) Ring Alert command.

2) Allocate Channel request.

3) channel Assignment command.

4) Make Link command.

5) Deallocate Channel request.

6) Base Station Memberships Available broadcast. The 8-bit data field ofthe broadcast contains the number of memberships currently available inthis base station community. This broadcast will occur at least onceevery 200 msec on each order wire channel.

7) Membership Enrollment request. Submitted on a CSMA basis by roaminghandsets seeking membership in a new community.

8) Enrollment Interview commands. Eight different commands, actually:three to get the 24-bit handset serial number, three to get the 24-bithandset telephone number, one to identify the previous membership cell,if any, and one to assign a 7-bit ID number to the handset, therebycompleting its acceptance into the new cell community.

9) Adjacent Cell Map broadcast. The 12 lease significant bits of thisbroadcast indicate, for each of 12 possible frequency cells, whetherthat cell is (1) adjacent to the current cell or (0) not adjacent to thecurrent cell.

10) Adjacent Cell Time Offset report. Three different reports, actually:one to indicate PN code phase offset, one to indicate symbol offsetwithin a subframe, and one to indicate subframe offset within a frame.The 8-bit data field of these reports indicates the particular offset,relative to the current cell, of the adjacent cell base stationidentified in the Handset ID field. These reports are submitted,initially on a CSMA basis, by any scouting or roaming handset, and arethen confirmed on an assigned TDMA basis.

11) Adjacent cell Time Offset broadcast. Three different broadcasts,actually: one to indicate PN code phase offset, one to indicate symboloffset within a subframe, and one to indicate subframe offset within aframe. The 8-bit data field of these broadcasts indicates the particularoffset, relative to the current cell, of the adjacent cell base stationidentified in the Handset ID field.

12) Voice channels Available broadcast. The 8-bit data field of thisbroadcast contains the number of currently unassigned voice channelswithin this base station. This broadcast will occur nominally once eachsecond.

13) CSMA Statistics broadcast. The 15 least significant bits of thisbroadcast contain CSMA slot capacity, loading, and collision statisticsfor the previous 1-second period.

14) Adjust Transmit Code Phase command. The 8-bit data field of thiscommand is a two's complement number indicating the handset transmitcode phase adjustment, in sixteenths of a chip to be advanced; thus avalue of -3 would indicate to retard the transmit phase of the handsetidentified in the Handset ID field by 3/16 of a chip. Data valuesoutside the range of -4 to +4 are ignored.

15) Adjust Transmit Power Level command. The 8-bit data field of thiscommand is a two's complement number indicating the handset transmitpower adjustment, in units of db gain; this value is essentially addedto the transmit power control bias term (see Section 4.3) of the handsetidentified in the handset ID field. Data values outside the range of -4to +4 are ignored.

CARRIER SENSE MULTIPLE ACCESS (CSMA) ISSUES

Handsets seeking entry to a cell (i.e., a base station) are unknownentities to the base station, thus the invention provides for thehandset to access the base station. Also, in order to accommodate otherasynchronous events (e.g., handset transition from STANDBY to ACTIVEmode and requesting allocation of a voice channel) and avoid the delaysinherent in a purely cyclical or polling approach, again, some othermeans is desirable.

A carrier sense multiple access (CSMA) approach seems well suited tosupporting these relatively infrequent demands, but it brings with itthe requirement to manage the CSMA resources intelligently. Severaldesign features have been incorporated in this regard.

First, the fraction of slots available for CSMA use will be arranged toprovide a suitable probability of no collision on the first accessattempt.

Second, the base station will maintain statistics of the use ofavailable CSMA slots and will broadcast these statistics to the handsetsfor use in making intelligent choices of initial access and backoffstrategies.

Third, the powerful parity check code included in inbound order wiretransmissions minimizes the possibility that when collisions do occurthey would not be recognized as such, thus the likelihood of the basestation erroneously interpreting the demodulated results of collidedtransmissions is extremely low.

Any CSMA access attempt which is not acknowledge within 35 msec will beconsidered to have failed, the appropriate backoff strategy will beselected, and a retry will be scheduled accordingly.

VOICE CHANNEL CONTROL DATA STRUCTURE AND PROTOCOL

Each voice channel burst contains a 2-symbol field allocated for channelcontrol, i.e., inband order wire functions such as handset transmitpower control, handset transmit code phase control, and other functionsto be identified. This provides a capacity of:

200 symbols/sec=128 symbols/frame

400 bits/sec=256 bits/frame

in each direction, inbound and outbound, for these purposes, so thathandsets with calls in progress still have access to full order wirefunctionality as described earlier.

Outbound channel control data is organized into 16-bit commands andacknowledgements formatted as shown in FIG. 11 and frame synchronized toprovide 16 such commands per frame (25 per second) per voice channel.

Each command is composed of a 6-bit function field and a 10-bit datafield. Unlike the order wire channel, no handset ID field is requiredsince the handset being addressed is implicit in he voice channelassignment.

Inbound channel control data is organized into 16-bit requests andacknowledgements formatted identically to outbound commands andsynchronized with them but offset by half a subframe. Inbound responsesto outbound commands commence three half-subframes after the commandtransmission is complete, and outbound responses to inbound requestscommence in the burst following completion of the request.

DETAILED SIGNAL PROCESSING OPERATIONS

The following describes the signal processing operations and sequencesutilized by the system to acquire and track the signal, maximize itsquality, demodulate data from it, determine when to transfer to anadjacent cell, and accomplish such transfers.

INITIAL SIGNAL ACQUISITION (HANDSETS ONLY)

When a handset is first powered on, it is assumed to have a prioriknowledge of its "home" base station PN code and frequency channel, butto have no knowledge of its time offset from that base station, and toknow to within only 9 KHz its frequency offset from nominal for thatchannel. (The frequency offset from nominal at the base station isassumed to be less than 100 hertz.)

The initial search resolves these time and frequency uncertainties byseeking to acquire the base station order wire signal at each of255*2=510 PN code phase uncertainty states and 19 frequency bins spaced1 KHz apart. Each of the resulting 19*510=9690 composite uncertaintystates is examined for 398.44 μsec (=one 255-chip PN sequence length),and since there are 3 correlators per receiver, a total of 9690*398.44μsec/3=1.29 sec would be required to complete the search if the signalwere constantly present.

Since the base station order wire signal is present only half the time,however, (the inbound signal being spread with a different PN code),each uncertainty state must be searched at least twice, once at time tand again t+(2n+1)*5 msec, so the total time required to acquire PN chipsync (to within 0.25 chip or so) and resolve frequency offset (to within500 Hz or so) is at least twice this, or 2.6 seconds.

If the peak power measure of all the uncertainty states is not at leastTBD db greater than the average of all the non-peak states, then it isassumed that the first attempt failed due to an antenna null, and thesearch process is repeated on the other antenna, for a worst case totalof 5.2 seconds.

Note again that subsequent acquisitions will in general be essentiallyinstantaneous, because the initial acquisition and carrier pull-in willhave removed all frequency uncertainty, and Adjacent cell Time Offsetbroadcasts will have eliminated most code phase and other timeuncertainties.

Note too that acquiring PN code phase sync automatically also achievesOW symbol sync, but an additional several frames will be required toachieve frame sync and carrier pull-in prior to being able to demodulatedata. These processes are described in the sections following:

SUBFRAME SYNCHRONIZATION (HANDSETS ONLY)

Subframe synchronization is achieved as follows (see FIG. 12):

1) Return the coder and the carrier frequency to the code phase andfrequency corresponding to the initial acquisition energy peak (with theorder wire signal still selected).

2) Observe 3 subframes of (I,Q) measures from the correlator, eachintegrated over one OW symbol; in particular, observe the power profileof the data (modulo 26 OW symbol times per subframe), determine the peakpower measure, and verify that it is at least 9 db above the average ofthe others. This corresponds to the onset of the outbound sounding burstat the start of each subframe.

This observation is accomplished by constructing a 26-element histogram,clearing all elements to zero, then adding to each the power measure ofthe corresponding (I,Q) sample (that is, sample number i mod 26, for i=0to 77), where the measure of power is defined as I 2+Q A2.

3) The histogram index j such that

    h(j)>h(i), all i/=j

and

    h(j)>Pavg+9 db

where:

    Pavg=(Ptot-h(j)-h(j+1 mod 26))/24

and

    Ptot=Sum(h(i),i=0, 77)

represents the delay, in OW-symbol increments, of the actual frame startrelative to the postulated frame start (i=0). If no such index j exists,then repeat steps (2) and (3) using the other antenna.

4) Set OW symbol count=(26j) mod 26. (OW symbol count will beincremented by 1 (modulo 26) on each subsequent OW symbol). Thiscompletes the frame sync process, so it may be disabled and the carrierand code tracking functions enabled.

ANTENNA SELECTION AND TRANSMIT POWER CONTROL (HANDSETS ONLY)

During each of the two sounding bursts at the start of each subframe(one burst received on each antenna), a power measurement is made andprojected to the midpoint of the inbound signalling period. The antennacorresponding to the larger projected power measure is selected to beused during the remainder of the subframe (both outbound and inboundportions). The larger projected power measure itself, plus a biascorrection term determined by the base station over a longer time frame,is used to set the power level for the inbound transmission (if any).Reference is made to the elements shown in FIG. 13.

The power is measured for each sounding burst as follows: (I, Q) samplesare input from the correlator and integrated in integrators coherentlyover 6 voice symbols; total power is then computed from theseburst-coherent (Ij, Qj) measures as

    ______________________________________                                        P1 = I1 2 + Q1 2      ; antenna 1                                             P2 = I2 2 + Q2 2      ; antenna 2                                             ______________________________________                                    

and projected to the midpoint of the inbound signalling period:

    ______________________________________                                        PWR1 = P1 + 0.75 * (P1 - P1')                                                                     ; proj = current +0.75*                                   PWR2 = P2 + 0.75 * (P2 - P2')                                                                     ; (current - previous)                                    P1' = P1; P2' = P2  ; set previous = current                                  ______________________________________                                    

Antenna Selection is then simply

    If PWR1>PWR2

then select antenna 1 (k=2)

else select antenna 2 (k=2)

The antenna selected algorithm is the same independent of whether a callis in progress on the handset.

The transmit power Pxmit for this subframe is then computed as

    Pxmit=Kp+Pref-log (PWRk)-Atten+Bias

where

Kp=nominal transmit power for log (PWRk)=Pref-Atten+Bias

Pref=reference receive power level.

Attn=attenuator setting set by AGC (see Section 4.9)

This bias correction term for each handset is determined at the basestation once each 64 frames as follows:

    Bias=Bias+K1d*log (Prcv/Pref)

where

    ______________________________________                                        Prcv = Pp from base station code phase tracking function (see                      Section 4.5.2).                                                               = Ip 2 + Qp 2, Ip and Qp integrated coherently over a                         12-1/8 symbol handset sync period                                        Pref = reference receive power level                                          ______________________________________                                    

and K1d is chosen to provide a loop bandwidth of 0.10 Hz. The transmitpower control algorithm is the same independent of whether a call is inprogress on the handset.

CARRIER PULL-IN AND TRACKING (HANDSETS ONLY)

Carrier pull-in and tracking are achieved using the AFC functiondescribed in the following, which is enabled on the first OW symbolcount of 0 following subframe sync. FIG. 14 exemplifies the frequencydiscriminator and AFC carrier tracking loop subsystem used in theinvention.

Base on the power measurements taken during the sounding bursts, ifPWR1>PWR2, then let k=1 (else k=2) and compute the discriminator Dafc as

    Dafc=adjust (phi2-phi1)

where

    phi1=a tan (Qk1, Ik1)

    phi2=a tan (Qk2, Ik2)

    adjust (×)=if abs(×)<pi then×else×-2*pi*sign (x×).

and the subscripts 1 and 2 denote samples taken during the first andsecond halves of each sounding burst, respectively.

Next, input Dafc to a first-order AFC loop

    df=df+K1a*Dafc±3450/pi

and output df+nominal, scaled appropriately, to the carrier NCO. Theloop is iterated at the subframe rate, i.e. 100 Hz and K1a is chosen toprovide a loop bandwidth of 6 Hz. The discriminator operates only onoutbound order wire sounding bursts and has a range of ±3450 Hz.

Carrier pull-in will be essentially complete within three loop timeconstants, or about 0.15 sec, so at that time the data demodulationfunction is enabled.

The carrier tracking function is the same, independent of whether a callis in progress on the handset.

CODE PHASE TRACKING

Code phase tracking is performed both at the handsets and at the basestations, but it is done differently in either place. This followingdescribes the code phase tracking algorithms both for handsets and forbase stations.

Code phase tracking is accomplished at the handsets using the delay lockloop function described following, which is enabled on the first OWsymbol count of 0 following subframe sync.

Base on the power measurements taken during the sounding bursts, ifPWR1>PWR2 then let k=1 (else k=2), and compute the discriminator Dco as

    Dco=(Pe-P1)/Pp

where

    Pe=(Iek1+Iek2) 2+(Qek1+Qek2) 2

    P1=(Ilk1+Ilk2) 2+(Qlk1+Qlk2) 2

    Pp=(Ipk1+Ipk2) 2+(Qpk1+Qpk2) 2

and the subscripts e, l, and p denote measures taken with the referencecode displaced 1/2 chip early and 1/2 chip late relative to nominal, andat nominal, respectively, and the subscripts 1 and 2 denote samplestaken during the first and second halves of each sounding burst,respectively.

Dco is then input to a first order delay lock loop

    dp=K1b*Dco/4

and the loop output dp is used to adjust the code phase in units of 1/16of a chip. The loop is iterated at the subframe rate, i.e. at 100 Hz,and K1b is chosen to provide a loop bandwidth of 6 Hz.

Note that the code phase tracking function is the same at each handset,independent of whether a call is in progress on that handset.

CODE PHASE TRACKING AT BASE STATIONS

In order to maximize the synchronicity of the inbound signals at eachbase station, the code phase at arrival is measured for each handset ineach community at the base station associated with that community. Thisprocess, illustrated in FIG. 16, is implement as follows:

Each handset has an associated 7-bit ID number which it receives fromthe base station at the time it joins that base station community.Handsets with ID numbers from 0 to 63 are implicitly associated withorder wire subgroup 0 of that base station; those with ID numbers from64 to 127 are implicitly associated with order wire subgroup 1. Eachorder wire channel must thus support up to 64 handsets.

During the Handset Sync portion of each inbound half subframe, thehandset whose ID number modulo 64 equals the number of the currentsubframe within the frame transmits a 121/8 symbol all-ones sync burst.The base station receives this burst and computes the discriminator Dco2as

    Dco2=(Pe-Pl)/Pp

where

    Pe=Ie 2+Qe 2

    Pl=Il 2+Ql 2

    pp=Ip 2+Qp 2

and the subscripts e, l, and p denote measures taken with the referencecode displaced 1/2 chip early and 1/2 chip late relative to nominal, andat nominal, respectively, and each of the I and Q inputs have beencoherently integrated over the full 121/8 symbol (388-chip) measurementperiod.

Dco2 is then input to a first order delay lock loop

    dp=K1c*Dco2/4

and the loop output dp is used to adjust the handset transmit code phasein units of 1/16 of a chip. This function is iterated at the subframerate, i.e. at 100 Hz, so for each handset, it's at the frame rate (640msec, or 1.56 Hz), and K1c is chosen to provide a loop bandwidth of 0.02Hz.

The loops are actually closed via communication with each handset, usingthe order wire channel for handsets with no call in progress or usingthe voice channel control field for handsets with calls in progress.Other than this difference, the code phase tracking function at the basestation is the same for each handset, independent of whether a call isin progress on the handset.

DATA DEMODULATION

Once its AFC loop has settled, a handset may begin to demodulate orderwire data and engage in order wire dialogs with the base station inorder to subscribe to and participate in the cell community as describedearlier. Once it has subscribed to a particular community or cell, itmay then receive and originate calls, initially via the order wirechannel but predominantly via a voice channel, which of course requiresvoice channel data demodulation as well.

The algorithm used to demodulate this data is a combination of blockphase estimation, which adjusts the phase of the received symbols foroptimum detection in the presence of phase and frequency offsets, anddifferential data decoding of the received symbols. This algorithm isapplied straightforwardly to the voice channel and with minormodifications to the order wire channel. For the voice channel, thealgorithm operates as shown in FIG. 4.6.1 and described as follows:

For each of the 91 symbols (Ij, Qj) following the sounding bursts (inthe handset) or the handset sync burst (in the base station), computethe equivalent symbols (14j, Q4j) (with the date removed) as

    (I2, Q2)=(Ij, Qj) 2

    (14j, Q4j)=(I2, Q2) 2.

Then initialize the block integrators and phase estimate as

    ______________________________________                                        SumI4 = Sum (14j, j=0,15)                                                                           ;block length                                           SumQ4 = Sum (Q4j, j=0,15)                                                                           ;= 16 symbols                                           Phi4 = atan (SumQ4, SumI4)                                                    Phi = -Phi4/4 + pi/4                                                          PhiO = Phi                                                                    ______________________________________                                    

and rotate the first 8 symbols (Ij,Qj), j=0,7, by Phi:

    (Ij,Qj)=(Ij, Qj)*(cos (Phi), sin (Phi)).

For the next 75 symbols (Ij, Qj), j=8,82, update the block integratorsand phase estimate and rotate the symbol accordingly:

    ______________________________________                                        SumI4 = SumI4+14(j+8)-14(j-8)                                                 SumQ4 = SumQ4+Q4(j+8)-Q4(j-8)                                                 Phi4 = atan (SumQ4,SumI4)                                                     Phi = -Phi4/4+pi/4+Ntrack*pi/2                                                (Ij,Qj) = (Ij,Qj)* (cos(Phi), sin(Phi))                                       PhiO = Phi                                                                    ______________________________________                                    

where Ntrack=0, 1, 2 or 3 such that ABS (Phi-PhiO) is a minimum, i.e.,so as to produce minimum rotation relative to the previous rotation.

Next, rotate the final 8 symbols (Ij,Qj),j=83,90, by the final value ofPhi:

    (Ij,Qj)=(Ij,Qj)*(cos (Phi), sin (Phi)).

Finally, quantize the rotated symbols to 00, 01, 10, or 11 according tothe sign of Ij and Qj

    (Ij,Qj)=(sign(Ij),sign(Qj)), j=0,90,

and input the result to the differential decoder as shown in FIG. 17.Symbols 1 through 90 of the decoder output are the demodulated data forthis burst. (Date to be transmitted are first differentially encoded asshown in FIG. 18.

For the order wire channel, the algorithm is essentially the same exceptthat the block length is 2 OW symbols rather than 16 voice channelsymbols, and the phase reference symbol is shorter than the other OWsymbols. Also, the frame sync portion of each order wire burst ishandled differently, namely as 7 voice channel symbols. Thus thealgorithm becomes:

For each of the 11 symbols (Ij,Qj) following the sounding bursts (in thehandset) or the handset sync burst (in the base station), compute theequivalent symbols (I4j,Q4j) with the data removed, as

    (I2,Q2)=(Ij,Qj) 2

    (14j,Q4j)=(12,Q2) 2.

Then initialize the block integrators and phase estimate as

    ______________________________________                                        SumI4 = Sum (14j,j=0,1)                                                                             ;block length =                                         SumQ4 = Sum (Q4j,j=0,1)                                                                             ; 2 OW symbols                                          Phi4 = atan (SumQ4,SumI4)                                                     Phi = -Phi4/4 + pi/4                                                          Phi0 = Phi                                                                    ______________________________________                                    

and rotate the first symbol (I0,Q0) by Phi:

    (I0,Q0)=(I0,Q0)*(cos (Phi), sin (Phi)).

For the next 10 symbols (Ij,Qj), j-1,10, update the block integratorsand phase estimate and rotate the symbol accordingly:

    ______________________________________                                        SumI4 = SumI4+I4(j+1)-I4(j-1)                                                 SumQ4 = SumQ4+Q4(j+1)-Q4(j-1)                                                 Phi4 = atan (SumQ4,SumI4)                                                     Phi = -Phi4/4+pi/4 +Ntrack *pi/2                                              (Ij,Qj) = (Ij,Qj)* (cos(Phi), sin(Phi))                                       Phi0 = Phi                                                                    ______________________________________                                    

where

Ntrack=0, 1, 2, or 3 such that ABS (Phi-Phi0) is a minimum, i.e. so asto produce minimum rotation relative to the previous rotation.

Next, rotate the 7 frame sync symbols (Ij,Qj),j=11,17, by the finalvalue of Phi:

    (Ij,Qj)=(Ij,Qj)*(cos (Phi), sin (Phi)).

Finally, quantize the rotated symbols to 00, 01, 10, or 11 according tothe sign of Ij and Qj

    (Ij,Qj)=(sign(Ij),sign(Qj)),j=0,17,

and input the result to the differential decoder. Symbols 1 through 10of the decoder output are the demodulated OW data for this burst.Symbols 11 through 17 of the decoder output are the demodulated framesync data for this burst. (OW data to be transmitted are also firstdifferentially encoded.)

SCOUTING, ROAMING, AND CELL TRANSFER

The system implements certain features to support rapid cell transfer.One of these is the maintenance and broadcast of a database of therelative time offsets of adjacent cell base stations. The information inthe database is supplied by handsets which acquire adjacent cell orderwire signals on a scouting or roaming basis.

Again, scouting activity is essentially roaming activity, but with theintent of gathering data about the surrounding environment, rather thanof actually transferring cell membership. Scouting handsets relay timeoffset information regarding adjacent cells back to the base station oftheir currently assigned cell; roaming handsets which transfer to anadjacent cell impart this information regarding previous cell timing tothe base station of the new cell.

The information so gathered is verified and broadcast by each basestation via the order wire channel and via the channel control portionof each active voice channel.

Scouting and roaming searches differ from initial searches primarily inthat they are more focussed, that is, they search at only a singlefrequency, namely the handset's current carrier tracking frequencywithin the current cell, and, at least initially, they search only a fewchips of PN code phase uncertainty (proportional to data staleness). Theother main difference is that carrier frequency, PN code phase, andpower level tracking operations are maintained on the original signalduring scouting and roaming searches.

SCOUTING

For scouting, if the more focussed search fails on both antennas, it isthen broadened to include all 255 PN chips code phase uncertainty. Ifeven this broader search fails on both antennas, the current scoutingeffort is terminated and normal operation within the current cell isresumed, without a scouting report (Adjacent cell Time Offset report)being submitted to the base station.

If any of the searches succeed, however, subframe and framesynchronization are also performed and a scouting report is submitted.

ROAMING AND CELL TRANSFER

Received power is measured once each subframe. A filtered average ofthis measure is also maintained so as to provide a 2-second timeconstant. Whenever this filtered average falls below a threshold definedby the signal level at which transfer to another cell becomes desirable,a roaming search is initiated, which searches first for the adjacentcell order wire signal most recently acquired.

If this focussed search fails on both antennas, a similar search isconducted on both antennas for the next most likely adjacent signal tobe acquired, and so on, until all adjacent signals have been searched.For each adjacent signal acquired, if the measured power level on thatsignal is greater than on the current signal, then the handset listensfor a Base Station Memberships Available broadcast.

If memberships are available (and, if a call is in progress on thehandset, if voice channels are also available), then the handset issuesa Membership Enrollment request. On verification of the enrollmentrequest, the base station conducts an enrollment interview with thehandset, and the transfer of the handset membership, to the adjacentcell base station is completed, along with any call in progress on thehandset.

SIGNAL PRESENCE MONITORING AT BASE STATIONS

In order to detect those situations in which a handset signal canreasonably be assumed to be lost, especially if it is currently assigneda voice channel and voice channels are currently in high demand, afiltered average of the received power from each of the handset syncperiods is maintained as:

    Fp(j)=(I-K1f)*Fp(j)+K1f*Prcv(j)

where

Prcv(j)=Ip 2+Qp 2, Ip and Qp integrated coherently over 121/8 symbols,

and where K1f is chosen to provide a time constant of 2 seconds.Whenever the Fp value for any handset j falls below a specified lowerthreshold, the handset is noted as being off-line; whenever its Fp valuereturns above an upper threshold, it is noted as being on-line.

Any call in progress on a handset determined to be off-line isterminated. Incoming calls whose destination handset is off-line aregiven a busy signal.

AUTOMATIC GAIN CONTROL (AGC) IN HANDSETS

In order to minimize the dynamic range requirements (and thus the powerand cost) of the signal-path A-to-D converter used in handsets, someform of automatic gain control (AGC) of the A-to-D input signal isrequired. FIG. 19 depicts the AGC approach selected for this system. Theconcept is as follows:

During each sounding burst, the analog input signal is correlated withthe reference PN waveform and coherently integrated over 6 symbols, thendumped to square-law devices SLD whose outputs are summed andlog-amplified, then converted to digital. This digital log-domain powermeasure is read by software at the end of each sounding burst. At theend of the second burst, the larger of the two power measures (Pmax) isselected by software to set the signal-path attenuation for theremainder of the current subframe and the sounding period of thefollowing subframe. The attenuation is determined as:

    Atten=Atten+Kpow*(Pmax-Plimit+6 dB)

where Kpow is a function of the log amplifier gain and attenuator gain.The attenuator setting is also used in the determination of the handsettransmit power setting for the current subframe.

For signal acquisition, the attenuator is set (separately for eachantenna and for each new code phase and carrier frequency uncertaintyrange scan) so that the rms noise level P0 is 18 db below the maximumA-to-D converter input level, thus:

    Attn=Attn+Kpow*(P0-Plim+18 db).

An embedded microcontroller or microprocessor can be used to control notonly the operational sequences involved in command handling, but thereare decided advantages to incorporating not only the sequence controlfunctions but much of the signal processing as well into a programmabledevice such as a digital signal processor. These advantages include:

reduced hardware design time, due to:

having fewer parts to incorporate no ASIC design time or fab lead timegreatly reduced FPGA complexity and design time;

increased flexibility to modify or fine-tune algorithms once the systemis already built and in test.

While a preferred embodiment of the invention has been shown anddescribed, it will be appreciated that various modifications andadaptations of the invention will be obvious to those skilled in the artand still be within the spirit and scope of the invention as set forthin the claims appended hereto.

What is claimed is:
 1. In a spread spectrum orthogonal code divisionmultiple access (OCDMA) communication system having at least one basestation and a plurality of user handsets in which a set of orthogonalRadamacher-Walsh (RW) function are overlaid with a pseudonoise (PN)sequence forming a coded spreading sequence for an information signal,each orthogonal function of said set carries voice data for a singleuser in said system, a source of selected carrier signals and means tomodulate said information signal on said carrier signal to form atransmit signal with said coded spreading sequence on said transmitsignal for radio broadcasting between said at least one base station andsaid user handsets, each said user handset having a time base and meansfor controlling its power transmission, and wherein the base stationtransmits a sounding signal, the method of controlling operation of saidhandsets, the improvement comprising:(1) measuring the time base errorof each handset at said base station and transmitting time basecorrection information from said base station to each handset andcontrolling the timing of operation of each handset thereby,respectively, and (2) measuring the power of the sounding signal andperforming the compensation of the handset transmit power usinginstantaneous power control of handset to base station link so as toavoid signal interference.
 2. In a spread spectrum orthogonal codedivision multiple access (OCDMA) communication system having at leastone base station and a plurality of user handsets in which a set oforthogonal Radamacher-Walsh (RW) functions are overlaid with apseudonoise (PN) sequence forming a coded spreading sequence for aninformation signal, each orthogonal function of said set carries voicedata for a single user in said system, a source of selected carriersignals and means to modulate said information signal on said carriersignal to form a transmit signal with said coded spreading sequence onsaid transmit signal for radio broadcasting between said at least onebase station and said user handsets, each said user handset having atime base and means for controlling its power transmission, and whereinthe base station transmits a sounding signal, the improved method ofcontrolling operation of said handsets comprising:(1) means formeasuring the time base error of each handset at said base station andtransmitting time base correction information from said base station toeach handset, respectively, (2) means for measuring the power ofsounding signal and performing the corresponding compensation of thehandset transmit power and means for using the measured power to eachhandset to instantaneously control transmitted power of handset to basestation link so as to avoid signal interference.
 3. In a spread spectrumorthogonal code division multiple access (OCDMA) communication systemhaving at least one base station and a plurality of user handsets inwhich a set of orthogonal Radamacher-Walsh (RW) functions are overlaidwith a pseudonoise (PN) sequence forming a coded spreading sequence foran information signal, each orthogonal function of said set carriesvoice data for a single user in said system, a source of selectedcarrier signals and means to modulate said information signal on saidcarrier signal to form a transmit signal with said coded spreadingsequence on said transmit signal for radio broadcasting between said atleast one base station and said user handsets, each said user handsethaving a time base and means for controlling it power transmission, eachhandset having a local oscillator, said base station having an orderwire channel for transmitting a reference sounding signal, theimprovement comprising:means for measuring the time base error of eachhandset at said base station and transmitting time base correctioninformation from said base station to each handset, respectively, andcorrecting said tune base using said tune base correction information,means for measuring the power of the reference sounding signal andperforming instantaneous compensation of the handset transmit power soas to avoid signal interference, and means at each handset for receivingsaid reference sounding signal and phase-locking said local oscillatorto said reference sounding signal.
 4. In a spread spectrum orthogonalcode division multiple access (OCDMA) communication system having atleast one base station and a plurality of user handsets in which a setof orthogonal Radamacher-Walsh (RW) functions are overlaid with apseudonoise (PN) sequence forming a coded spreading sequence for aninformation signal, each orthogonal function of said set carries voicedata for a single user in said system, a source of selected carriersignals and means to modulate said information signal on said carriersignal to form a transmit signal with said coded spreading sequence onsaid transmit signal for radio broadcasting between said at least onebase station and said user handsets, each said user handset having atime base and means for controlling its power transmission, and whereinthe base station transmits a sounding signal, each said handset having alocal oscillator, the method of controlling operation of said handsetscomprising:measuring the time base error of each handset at said basestation and transmitting time base correction information from said basestation to each handset, respectively, using instantaneous power controlof handset to base station link, including measuring the power of thereference sounding signal and performing the compensation of the handsettransmit power so as to avoid signal interference, transmitting areference sounding signal from said base station, receiving saidreference sounding signal, and phase-locking said handset localoscillator to said reference sounding signal.